In the design of modern transportation vehicles, structural vibration and interior noise have become important problems that must be addressed. For example, control of sound transmission into enclosed spaces is an important issue for helicopter systems. Various studies have shown that the predominant frequency components associated with the noise transmission lie in the frequency range of 50 Hz to 5500 Hz. There are a number of approaches that may be used to control a sound field within a helicopter cabin.
Among the different approaches, one is based on controlling the radiation (transmission) from (through) a flexible structure by active means which is referred to as Active Structural Acoustic Control (ASAC). The ASAC scheme, which is an effective solution for low-frequency applications, takes advantage of vibrating structural elements as secondary noise sources to cancel the sound fields generated by a primary noise source (A. Sampath, et al., “Active Control of Multiple Tones Transmitted in an Enclosure”, Journal of the Acoustical Society of America, Vol. 106, No. 1, Pages 211-225, July 1999; M. Al-Bassyiouni, et al., “Zero Spillover Control of Enclosed Sound Fields”, SPIE's Annual International Symposium of Smart Structures and Materials, Newport Beach, Calif., March 4-8, Vol. 4362, Paper No. 4326-7, 2001; and, M. Al-Bassyiouni, et al., “Experimental Studies of Zero Spillover Scheme for Active Structural Acoustic Control Systems”, Proceedings of the 12th International Conference on Adaptive Structures and Technologies (ICAST), University of Maryland, College Park, Md., Oct. 15-17, 2001). It appears that ASAC schemes require much less dimensionality than Active Noise Control (ANC) schemes in order to realize widely distributed spatial noise reduction. As is known in the art, ANC schemes are generally used to minimize noise by using various cancellation techniques. However, active research is still being pursued to address issues such as sensors, actuators, and control architecture.
Fiber-optic sensors have the advantages of being lightweight, having high sensitivity, and being multiplexible. Since the original demonstrations showed that optical fibers could be used as acoustic sensors (Bucaro J. A., et al., “Fiber Optic Hydrophone”, Journal of Acoustical Society of America, 62, Pages 1302-1304, 1977; and, Cole, J. H., et al., “Fiber Optic Detection of Sound”, Journal of Acoustic Society of America, 62, Pages 1136-1138, 1977), substantial research has been carried out in this field. Much of this effort has been directed towards the development of hydrophones for ultrasonic detection which does not suit the needs of an ASAC system.
Since Bragg grating sensors were shown to be multiplexible by using Wavelength Division Multiplexing (WDM) techniques, Baldwin, et al., (“Bragg Grating Based Fabry-Perot Sensor System for Acoustic Measurements”, Proceedings of the SPIE 1999 Symposium on Smart Structures and Materials, Newport Beach, Calif., Mar. 1-5, 1999), developed a Bragg grating based Fabry-Perot sensor system for use in ASAC schemes. However, the sensor bandwidth was found to be limited, and in addition, the sensor was found to have low sensitivity due to the high Young's modules of silica resulting in “acoustically induced strains” which also limits the application of this type of sensors.
Hence, low finesse Fabry-Perot sensors have become attractive choices for high performance sensing in this area. For example, a Fabry-Perot optical sensing device for measuring a physical parameter, described in U.S. Pat. No. 5,392,117, comprises a Fabry-Perot interferometer through which a multiple frequency light signal having predetermined spectral characteristics is passed. The system further includes an optical focusing device for focusing at least a portion of the light signal going outwards from the Fabry-Perot interferometer and a Fizeau interferometer through which the focused light signal is passed.
The Fabry-Perot interferometer includes two semi-reflecting mirrors substantially parallel to one another and spaced apart so as to define a Fabry-Perot cavity having transmittance or reflectance properties that are effected by a physical parameter such as pressure, temperature, refractive index of a liquid, etc., and which causes the spectral properties of the light signal to vary in response to the changes in physical parameters.
The Fabry-Perot interferometer is provided with at least one optical fiber for transmitting the light signal into the Fabry-Perot cavity and for collecting the portion of the light signal being transmitted outwards. The Fizeau interferometer includes an optical wedge forming a wedge-profile Fizeau cavity from which exits a spatially-spread light signal indicative of the transmittance or reflectance properties of the Fabry-Perot interferometer.
Of particular interest are sensor configurations that may be used for various measurements, such as for instance, measuring displacement, sound pressure, acceleration, as well as pressure gradient, air particle velocity, and acoustic intensity. Currently, there are no commercially available micro-scale fiber optic sensor systems that can be used for these measurements, since the current technology is generally directed toward condenser microphones.
Velocity sensors have numerous advantages, some of which are as follows: (1) better sensitivity to spherical waves compared to the sensitivity of a pressure microphone; (2) can be used along with the pressure microphones to measure the sound energy density; and (3) can be used along with pressure microphones to develop a unidirectional microphone that would favor waves incident from only one direction and discriminate waves incident from other directions.
The concept of a typical velocity microphone is known in the prior art and is based on measuring acoustic pressure by pressure condenser microphones. However, the complexity and bulkiness of known velocity microphones makes them difficult to use effectively in ASAC systems.
A new technology that has been introduced recently by Microflown, a Dutch company, which permits use of small scale air particle velocity sensors. However, these sensors make use of a thermal effect, and they disadvantageously need high operating temperatures. It is thus clear that a velocity sensor free of the disadvantages of prior art velocity sensors is still needed for industrial applications.
As part of ASAC systems as well as Active Vibration Control (AVC) systems, acceleration sensors of high sensitivity and low mass may also contribute to overall control of structural vibration and interior noise. In one manifestation, the conventional accelerometer consists of an inertial mass attached to a spring and this combination is located inside a housing that is exposed to the disturbance. In another manifestation, the accelerometer consists mainly of a uniform cantilever beam fixed to an accelerometer housing, which in turn is attached to the structure, the parameters of which are to be detected and measured. As the accelerometer vibrates due to base excitation, the cantilever tip oscillates about the undeformed axis, and the deflection at any point along the undeformed axis of the beam is a function of the excitation acceleration. It would be desirable to apply principles of the fiber tip based Fabry-Perot sensor to measurements of such a deflection of the oscillating beam.
Summarizing the discussion of the prior art supra, it is readily understood to those skilled in the art that it is still a long-lasting need in the field of active structural acoustical control and other fields mentioned previously to provide a wide bandwidth (in the frequency range of 50 Hz to 7.5 KHz and better) fiber tip based micro-optical sensor systems for various acoustic and vibration measurements, which are free of the disadvantages of prior art acoustics and vibration measurement systems.